Middlesex District Attorney Marian Ryan said Wednesday that there’s been “great progress” made in the investigation of a three-year-old triple murder that has been connected to the Marathon bombings, but she declined to provide any details.

Three men were killed in a Waltham apartment on Sept. 11, 2011. The case was investigated by the Middlesex DA’s office, but no one was ever charged.

After the Boston Marathon bombings, it was learned that suspect Tamerlan Tsarnaev was a close associate of one of the victims and a frequent visitor to the Waltham apartment. Friends of the victims complained to the press about the lack of progress in the case. Some said they told investigators about the Tsarnaev connection, and were surprised Tsarnaev was never interviewed about the case.

Asked about the murder at a candidates debate in Framingham Wednesday, Ryan said she was involved in the triple murder investigation in her role as a senior manager in the DA’s office, but was not directly responsible for it, until her appointment as district attorney in April 2013. She said it is still a “very active” investigation, and she cannot reveal any of its findings because it might tip off a suspect.

Some cases take a long time to break, Ryan said. Prosecutors are still assigned to the triple murder case, she said, but they have other cases assigned to them as well.

Asked to comment on speculation the FBI might have told the DA’s office to back off from the investigation, Ryan said “no one told me.”

Michael Sullivan, who is challenging Ryan in the Sept. 9 Democratic primary, was critical of the DA’s investigation, and about a lack of transparency in general.

“There’s already been one death as a result of the investigation, in Florida,” he said, referring to the death of Ibragim Todashev, a friend of Tamerlan Tsarnaev, while he was being interrogated by FBI and state police in his Florida home.

Sullivan implied the Marathon bombings might have been averted if Middlesex investigators had “followed the phone records” of the Waltham victims, which would have led them to Tsarnaev.

Ryan bristled at the suggestion a more thorough investigation might have prevented the attack at the 2013 Marathon finish line, and said responsible public officials must never deal in “speculation.”

Sullivan pledged to “move forward swiftly” on the investigation and be more forthcoming about releasing details.

Rick Holmes is Massachusetts political editor for GateHouse Media. He can be reached at rholmes@wickedlocal.com.